UNITED NATIONS  A population clock at U.N. headquarters hit 6 billion Tuesday and started racing toward 7 billion as an anxious world pondered what the new millennium holds for an increasingly crowded planet.

U.N. Secretary-General Kofi Annan welcomed the symbolic Baby 6 Billion in Sarajevo, Bosnia-Herzegovina  a boy born to refugee parents in a region returning to life after a decade of war.

Adnan Nevic's birth "should light a path of tolerance and understanding for all people," Annan said.

But the tens of thousands of children born on the Day of 6 Billion are far more likely to face lives of poverty and illiteracy in developing countries. UNICEF head Carol Bellamy noted that 1 in 3 children in the world's poorest countries won't live beyond age 5.

The challenge facing the world, Annan said, is to find "the will" to feed, clothe and house every inhabitant of Earth.

In Washington, meanwhile, President Clinton said the world's nations must work harder to erase the grinding poverty in which many of their people spend their lives.

"We must refuse to accept a future in which one part of humanity lives on the cutting edge of a new economy, while another part lives on the edge of survival," he said.

U.N. demographers had determined that the population would hit 6 billion on Tuesday, a doubling of the Earth's inhabitants in less than 40 years.

The population clock in the visitors lobby at U.N. headquarters was racing so fast Tuesday morning that it skipped from 5,999,999,998 to 6,000,000,001.

"Somebody had triplets," quipped one U.N. official.

The clock was inside a display set up by the U.N. Population Fund, which has advocated the right of people to determine the size of their families.

It is campaigning to fulfill the goal of the 1994 U.N. population conference  to provide basic education for all children, especially girls, by 2015, since research shows that educated women have fewer children.

With more than 1 billion people 15 to 24 just entering their reproductive years, the next population milestone will depend on the decisions they make about family size.

"Their choices will determine how many people will be on the planet by 2050 and beyond," read the Population Fund display.

And there is another "youthquake" coming, with 1.8 billion people under the age of 15.

Even with fertility rates falling, the United Nations projects that by 2050, the world's population will be between 7.3 billion and 10.7 billion, with 8.9 billion the most likely figure.

Chinese Premier Zhu Rongji said the birth of the 6 billionth human being would have occurred four years ago if it wasn't for the country's one-child policy. In a letter to a national meeting Tuesday on population controls, he said the strict limit on family size saved the world 300 million births.

But local officials have been accused of using threats, sterilizations and late-term abortions to meet stringent birth quotas, although the government denies it condones such tactics.

With a population of 1.25 billion, China is the world's most populous country. But India is set to hit the 1 billion mark next year, and U.N. demographers predict it will become the most populous nation by 2040.

Tuesday's announcement that India's population will be 1 billion on May 11, 2000  the anniversary of its multiple nuclear tests  provided a stark reminder of the choice between feeding millions and becoming a nuclear power.

While humanity is adding about 78 million people a year  more than 200,000 a day  the number of older people also is increasing dramatically in both developed and developing countries.

The Population Fund's Richard Leete said the aging of the world's population is raising questions for the 21st century: Should retirement ages be raised? Will people in industrialized countries be forced to work longer because their pensions won't sustain their retirement? What should be done to keep older people active mentally and physically?